In semiconductor fabrication processes, a semiconducting wafer must be repeatedly coated with many different material layers and processed in physical or chemical processes before the final IC device is fabricated. One of such numerous coating processes is the coating of a spin-on-glass layer as a planarization layer on top of a semiconductor structure.
Spin-on-glass (SOG) is frequently used for gap fill and planarization of inter-level-dielectrics (ILD) in multi-level metalization structures. It is suitable for low-cost fabrication of IC circuits. Most commonly used SOG materials are of two basic types; an inorganic type of silicate based SOG and an organic type of silicon oxide based polysiloxane which is featured with radical groups replacing or attaching to oxygen atoms. Based on these two basic structures, the molecular weight, the viscosity and the desirable film properties of SOG can be modified and adjusted to suit the specific requirements of an IC fabrication process.
SOG film is typically applied to a pre-deposited oxide surface as a liquid to fill gaps and steps on the substrate. Similar to the application method for photoresist films, a SOG material can be dispensed onto a wafer and spun with a rotational speed which determines the thickness of the SOG layer desired. After the film is evenly applied to the surface of the substrate, it is cured at a temperature of up to 400.degree. C. and then etched back to obtain a smooth surface in preparation for a capping oxide layer on which a second interlevel metal may be patterned. The purpose of the etch-back step is to leave SOG between metal lines but not on top of the metal, while the capping oxide layer is used to seal and protect SOG during further fabrication processes. The siloxane based SOG material is capable of filling 0.15 micron gaps and therefore it can be used in 0.25 micron technology.
When fully cured, silicate SOG has similar properties like those of silicon dioxide. Silicate SOG does not absorb water in significant quantity and is thermally stable. However, one disadvantage of silicate SOG is the large volume shrinkage during curing. As a result, the silicate SOG retains high stress and cracks easily during curing and further handling. The cracking of the SOG layer can cause a serious contamination problem for the fabrication process. The problem can sometimes be avoided by the application of only a thin layer, i.e., 1000.about.2000.ANG. of the silicate SOG material.
The curing of the SOG coating layer is frequently conducted in the same apparatus that spin-coats the material. The close proximity of the curing apparatus to the coating apparatus becomes a necessity when a thick SOG layer must be coated for a specific purpose, for instance, as a planarization layer. When a thick SOG coating is required, the material must be coated in multiple passes on the wafer and after each thin layer coating, the coating must be cured before the next layer of SOG may be added on top. It is therefore desirable to design an apparatus wherein the wafer may be coated with a thin SOG layer, then cured in a curing station, before it returns to the coating station for the next layer coating process.
A conventional SOG curing apparatus, that is used in conjunction with a coating apparatus in close proximity, is shown in FIG. 1. The curing apparatus 10 is constructed with an oven body 12 and a cover 14 for forming an enclosure, or a cavity 16 therein. The oven body 12 is normally formed in an elongated shape with an open top and a cavity 16 defined by two side panels 18,20, two end panels 22,24 and a bottom panel 26. The two end panels 22, 24 each has slot openings 30 provided therein for a pair of wafer carrying blades 32, 34 to pass therethrough.
The oven body 12 further includes a plurality of metal heating blocks 36 (or hot plates) situated in the cavity 16 arranged in a matrix form with a preset spacing 38 therein-between. The present spacing 38 in the longitudinal direction is smaller than that in the transverse direction such that the pair of wafer carrying blades 32, 34 fit snugly therein. The smaller spacing 38 in the longitudinal direction is necessary such that, as shown in FIG. 1, three separate metal heating blocks form a single heat station with a minimal space therein-between. As shown in FIG. 1, B1, B2 and B3 are three separate heating stations each formed by three metal heating blocks 36. A nitrogen purge gas is flown into the oven body 12 through a vent outlet 40 to improve the heat distribution in the cavity 16 when the cover 14 is closed on the oven body 12. The purge gas of nitrogen exits the cavity 16 through a bottom portion of the oven body 12, i.e., through a vent outlet 42.
A plain view of the multiplicity of heating blocks 36, a pair of wafer carrying blades, 32, 34 (known as walking beam in the industry) which are guided by two pairs of rollers 44 are shown in FIG. 2. Within each heating station of B1, B2 or B3, the preset spacing 38 between the heating blocks 36 is kept at a minimum such that a uniform temperature of the surface of the heating blocks 36 within the heating station can be achieved. The preset spacing 38 is therefore maintained at a very small value, such as between about 3 mm and about 5 mm which is the distance between the wafer carrying blade 32 and the heating block 36.
A processing difficulty frequently arises due to the small preset spacing 38 utilized in the SOG curing apparatus 10. Since both the heating blocks 36 and the wafer carrying blades 32, 34 are fabricated of a metal such as aluminum, when the preset spacing is not kept, friction occurs when the carrying blades 32, 34 touch the heating blocks 36. The mechanical friction between the two metal parts generates metal particles, i.e. aluminum particles, that can be a serious contamination source to the wafer that is being processed in the curing apparatus 10.
It is therefore an object of the present invention to provide a hot plate oven that can be used for curing a wafer coating layer without the particle contamination problem occurring in a conventional curing apparatus.
It is another object of the present invention to provide a hot plate oven for curing a coating layer on a wafer without particle contamination wherein the oven consists of a pair of wafer carrying blades that move in-between a plurality of metal heating blocks.
It is a further object of the present invention to provide a hot plate oven for curing a coating layer on a wafer without generating particle contamination which is equipped with an interlock circuit for shutting down the wafer transporting function in the oven when any touching between the wafer carrying blades and the heating blocks is detected.
It is another further object of the present invention to provide a hot plate oven for curing a coating layer on a wafer without generating particle contamination by providing an interlocking circuit which detects an electrical short between the wafer carrying blades and the metal heating blocks and thereby shutting down the curing process.
It is still another object of the present invention to provide a method for preventing particle contamination in a hot plate oven for curing a wafer coating layer by incorporating an interlock circuit for shutting down the curing operation when a contact between a wafer carrying blade and a metal heating block is detected.
It is yet another object of the present invention to provide a hot plate oven for curing a coating layer on a wafer without generating particle contamination which consists of an interlocking circuit formed by a solid state relay, a resistor, a relay, a reset switch and a buzzer alarm.